Temporarily immiscible dewaxing

ABSTRACT

A dewaxing process is described wherein a waxy oil stock is introduced into a cooling zone and contacted therein with dewaxing solvent system comprising two or more solvents. The solvents are introduced into the cooling zone in incremental amounts. The composition of the solvents introduced into the cooling zone is adjusted and the temperature in said cooling zone is gradually reduced, in order to produce temporary periods of solvent-oil immiscibility within said cooling zone interspersed with periods of miscibility. The solvent admixture is cooled and agitated as it passes through the cooling zone thereby precipitating at least a portion of the wax therefrom and forming a solvent-oil mixture containing precipitated wax. The mixture is subsequently removed from the cooling zone and oil of diminished wax content recovered therefrom. In one embodiment of the invention, the cooling zone comprises a plurality of stages and a high level of agitation is maintained within at least a portion of the solvent-containing stages thereby providing substantially instantaneous mixing of the solvent and the oil therein. The process is particularly useful for dewaxing heavy petroleum oil feedstocks, which have previously been subjected to a predilution operation.

Unite States Patent [191 Shaw [451 Mar. 18, 1975 1 1 TEMPORARILY IMMISCIBLE DEWAXING [75] Inventor: David H. Shaw, Sarnia, Ontario,

Canada [73] Assignee: Exxon Research and Engineering Company, Linden, NJ.

[22] Filed: June 22, 1973 [21] Appl. No.: 372,775

[52] U.S. CI. 208/33 [51] Int. Cl Cl0g 43/08 [58] Field of Search 208/33 [56] References Cited UNITED STATES PAT ENTS 2,223,939 12/1940 Jones 208/33 2,410,483 11/1946 Dons et al 208/33 2,625,502 1/1953 Backlund et a1 208/33 2,760,904 8/1956 Ford 208/33 3,350,297 10/1967 Torobin 208/33 3,681,230 8/1972 Eagen et a1. 208/33 Primary Examiner-Herbert Levine Attorney, Agent, or Firm-A. D. Litt; E. M. Corcoran [57] ABSTRACT A dewaxing process is described wherein a waxy oil stock is introduced into a cooling zone and contacted therein with dewaxing solvent system comprising two or more solvents. The solvents are introduced into the cooling zone in incremental amounts. The composition of the solvents introduced into the cooling zone is adjusted and the temperature in said cooling zone is gradually reduced, in order to produce temporary periods of solvent-oil immiscibility within said cooling zone interspersed with periods of miscibility. The solvent admixture is cooled and agitated as it passes through the cooling zone thereby precipitating at least a portion of the wax therefrom and forming a solventoil mixture containing precipitated wax. The mixture is subsequently removed from the cooling zone and oil of diminished wax content recovered therefrom. In one embodiment of the invention, the cooling zone comprises a plurality of stages and a high level of agitation is maintained within at least a portion of the solvent-containing stages thereby providing substantially instantaneous mixing of the solvent and the oil therein. The process is particularly useful for dewaxing heavy petroleum oil feedstocks, which have previously been subjected to a predilution operation.

24 Claims, 6 Drawing Figures SEMI? 70R TEMPORARILY IMMISCIBLE DEWAXING BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a process for dewaxing waxy petroleum oil stocks. Particularly, the process involves dewaxing the oil stock in the presence of a dewaxing solvent system comprising two or more solvents. More particularly, the process comprises introducing an oil stock and dewaxing solvents into a cooling zone, incrementally adding additional solvents, and controlling the temperature therein and the composition of solvent introduced so as to provide temporary periods of solventoil immiscibility, interspersed with periods of solventoil mixcibility.

2. Description of the Prior Art It is known in the prior art to dewax petroleum oil stocks by cooling an oil-solvent solution in scraped surface exchangers. In this process, the oil and selective solvent are admixed at a temperature sufficient to effect complete solution of the waxy oil in the solvent. The extent of dilution is dependent upon the particular oil and solvent employed and is adjusted to facilitate easy handling and optimum filtration rates and oil yields. The solution is cooled uniformly at a slow rate, e.g., l-5F. per minute, under conditions which are controlled so as to avoid any substantial agitation of the solution during precipitation of the wax. Notwithstanding the carefully controlled conditions used in this type of process, there are several deficiencies which hamper successful commercial operation. Most significant among these deficiencies is the loss of good heat transfer due to wax deposition on the exchange surfaces. Such failing has been repeatedly noted after short periods of operation, e.g., 24 to 48 hours. Associated directly with the loss of good heat transfer is the loss of careful control over the cooling rate and the corresponding loss of uniform crystal growth. The nonuniform crystal growth then results in lower filtration rates. The high pressure drop through the chilling section also reduces the maximum feed rate obtainable. Physical mashing of the wax crystals by the action of the scrapers may also contribute to poor filtration.

It is also known in the prior art to dewax petroleum oil stocks by cooling them in scraped surface exchangers using incremental solvent addition technique. In this process, the solvent is added at several points along the chilling apparatus. The waxy oil is chilled without solvent until some wax crystallization occurs and the mixture thickens considerably. The first solvent increment is then introduced and cooling continues. Each incremental portion of solvent is added as necessary to maintain fluidity until the desired separation temperature is reached, at which point the remainder of the solvent desired for filtration is added.

In this technique, it is well known and has been repeatedly demonstrated that the temperature of each increment of solvent should be the same as that of the main stream at its point of addition. Having the solvent at a lower temperature causes shock chilling of the slurry at that point, with resulting formation of crystal fines and impairment of filter rate; having the solvent warmer throws unnecessary additional load on the scraped surface chillers. It should be clearly understood that all of the chilling of the slurry in this well known de-waxing process is accomplished through the walls of the scraped surfacechillers, rather than by means of cold solvent. Dewaxed oil yields are higher on some stocks vis-a-vis the first cited process, but otherwise this process suffers from the same disadvantages. It is also known in the art as described in U.S. Pat. No. 2,361,503, to subject lubricating'oil fractions to agitation in a multi-staged tower with water or brine, which also serves as the cooling medium. This process suffers from the disadvantage that the cooling medium is completely immiscible with the wax and oil during the entire course of the cooling operation and rapid separation occurs between the feed and water unless the mixture is maintained in a highly agitated state or is emulsified. The use of water as a cooling medium practically limits the process to an upflow operation.

In U.S. Pat. No. 2,410,483, a two-staged dewaxing process is described in which a wax-containing oil stock, is heated above its cloud point and introduced into an elongated cooling vessel divided into a plurality of stages. Dewaxing solvent is injected into each of the stages, thereby cooling the oil. The resultant oil-solvent mixture is withdrawn from the cooling vessel at the temperature of the cloud point of the oil or no more than about 10F. above the cloud point. This mixture is then introduced into a pipe wherein additional cold dewaxing solvent is introduced. The mixture is cooled 'below its cloud point in the substantial absence of any agitation, thereby precipitating at least a portion of the wax therefrom. As a result of no agitation or substantially no agitation being provided during the crystallization phase of the dewaxing process, inferior filtration rates vis-a-vis other dewaxing processes, are experienced. In addition, the process requires at least two separate units, which, from an economic standpoint, is undesirable.

In co-pending U.S. Patent application Ser. No. 129,973, filed Mar. 31, 197] now U.S. Pat. No. 3,773,650, there is disclosed a dilution chilling dewaxing method in which a waxy oil stock is introduced into a cooling zone divided into a plurality of stages. Dewaxing solvent is introduced into the cooling zone at a plurality of points along the cooling zone, coming into contact with the oil and forming a wax-oil mixture. High levels of agitation are provided in at least a portion of the solvent-containing stages, thereby providing substantially instantaneous mixing of solvent and oil, i.e., within a second or less. As the coil passes through the cooling zone, it is cooled to a temperature sufficient to precipitate at least a portion of the wax therefrom, resulting in the formation of a wax slurry wherein the wax has a unique crystal structure with markedly superior filtering characteristics and wherein the wax slurry has a relatively high filtration rate and good dewaxed oil yields are obtained. While the process there disclosed and claimed overcomes several of the disadvantages of the prior art, there is still a desire for obtaining even higher filtration rates and greater oil yields thereby further reducing operation costs.

In co-pending U.S. Patent application Ser. No. 234,405, filed Mar. 10, 1972 now U.S. Pat. No. 3,779,894, there is disclosed another dilution chilling dewaxing operation in which the solvent is inthe solvent and the oil are miscible. The miscible solvent/oil mixture is then cooled and agitated, thereby precipitating at least a portion of the wax from the oil and a miscible oil/solvent mixture containing the precipitated wax is withdrawn from the chilling zone. This process represents considerable improvement over the prior art, but there is room for still further improvement, particularly in the dewaxing of heavy petroleum oil stocks.

SUMMARY OF THE INVENTION In accordance with the invention, it has now been discovered that waxy petroleum oil stocks can be efficiently dewaxed by use of the process of the subject invention.

Specifically, the process comprises introducing a wax-oil mixture at a temperature above its cloud point into av cooling zone, said mixture containing at least a portion of the wax dissolved therein, and contacting the mixture, therein, as it passes through said cooling zone, with a dewaxing solvent system comprising two or more solvents. The dewaxing solvents are introduced into the cooling zone in a plurality of increments. The composition and amount of each solvent increment introduced into the cooling zone are adjusted and the temperature within the cooling zone is gradually reduced, thereby producing a plurality of temporary periods of solvent- /wax-oil mixture immiscibility within the cooling zone, interspersed with periods of solvent/wax-oil mixture miscibility, achieved by introduction of additional incremental amounts of appropriate solvents. The resulting wax-oil/solvent mixture is cooled and agitated as it passes through the cooling zone thereby precipitating at least a portion of the wax from the oil and forming a solvent/oil mixture containing precipitated wax. The mixture is subsequently withdrawn from the cooling zone and an oil of diminished wax content is recovered therefrom.

Although the instant process of producing temporary periods of solvent/oil immiscibility within a cooling zone can be utilized in conventional cooling processes, such as dewaxing in scraped surface exchangers, the process finds specific utility in the dilution chilling dewaxing process mentioned above which is more fully described in co-pending US. Patent application Ser. No. 129,973, filed Mar. 31, 1971, and in US. Pat. Nos. 3,642,609, issued Feb. 15, 1972, and 3,644,195, issued Feb. 22, l972, the disclosures of which are incorporated herein by reference.

When using dilution chilling in conjunction with the process of the subject invention, the operation comprises introducing a wax/oil mixture into a cooling zone divided into a plurality of stages and contacting the mixture therein, as it passes from stage to stage of said cooling zone, with one or more solvents introduced into at least a portion of the stages by incremental addition along the length of the cooling zone. The temperature of the solvent-oil mixture is gradually reduced as it passes through the cooling zone and the amounts and composition of the solvent introduced into the chilling zone are adjusted so as to produce a plurality of temporary periods of solvent/oil immiscibility within at least a portion of the stages. Additional solvent is added in order to provide interspersed periods of solvent/oil miscibility. A high degree of agitation may be maintained in at least a portion of the solvent-containing stages thereby producing substantially instantaneous mixing of solvent and oil and the solvent/oil mixture is cooled to a temperature sufficient to precipitate at least a portion of the wax contained therein. Thereafter, an oil of diminished wax content is recovered. Utilization of temporary periods of immiscibility within the cooling zone, during the course of the cooling process, results in part, in high filtration rates, which, from an economic standpoint, are desirable.

Although the exact process mechanism is not known, it is postulated that at the point of oil-solvent immiscibility, separation takes place resulting in relatively large oil-rich droplets dispersed within a solvent-rich phase and wax crystals, which begin to form during the initial stages of cooling, dispersed within the oil-rich droplets. The high agitation levels within the various stages of the cooling zone provide a high degree ofturbulance causing dispersion of the oil-rich droplets into smaller droplets, but within each droplet, there are relatively low shear conditions favoring the agglomeration of the wax crystals present. Further chilling crystallizes more wax on these agglomerates, cementing them into stable structures. lmmiscibility is followoed by a period of solvent-oil miscibility to permit the leaching of oil from the agglomerates, thereby providing for high oil yields. It is apparent that the composition of each solvent increment'introduced into the chilling zone and the level of agitation within a particular stage must be closely controlled in order to give an optimum number and size of oil droplets, thereby facilitating the form ation of desirable wax crystal structures.

It is further apparent that the dilution chilling process, hereinabove described, is ideally suited to obtaining the benefits of this technique. The multiple stage cooling apparatus used in dilution chilling facilitates the use of large numbers of solvent increments providing considerable flexibility for maintaining the desired degree of immiscibility in specific stages. In addition, the use of controlled agitation in the dilution chilling process, facilitates the desired amounts and size of oil droplets.

The choice of solvents will depend upon a number of factors, such as the nature of the waxy oil feedstock, rate of cooling, etc. The initial contact of waxy oil feedstock and dewaxing solvent may result in either a miscible or an immiscible composition. As an example, this initial solvent contact may result in a miscible composition which, for example, by lowering of temperature and addition of another solvent, will result in the first period of immiscibility. Alternatively, the initial contact of the feedstock may be with a solvent which results in an immiscible composition. To this composition there would be added sufficient amount of an appropriate solvent in order to obtain a miscible composition. Regardless, however, of whether this initial contact results in a miscible or an immiscible composition, the principle of alternating periods of miscibility and immiscibility remains the same.

The feedstock that is used in the subject process may be any waxy petroleum oil stock or distillate fraction. In general, these oil stocks or distillate fractions will boil in the range of about 500 to above about l,300F. at atmospheric pressure. Preferred oil stocks are the lubricating oil and specialty oil fractions boiling within the range of about 600 and 1,300F. These fractions may come from any crude source such as the paraffmic crudes obtained from Aramco, Kuwait, the Panhandle, North Louisiana, Tiajuana and the like.

The process finds particular applicability with the heavy waxy oil stocks, which are characterized by having a viscosity greater than about 100 SUS at 210F. with more than percent of material boiling above about 800F. and more than 50 percent of material boiling below about 1,150F. at atmospheric pressure.

In the past it has been found that dewaxing of these heavy feedstocks, even in dilution chilling, was not efficient. However, it was discovered, as disclosed in copending U.S. Patent application Ser. No. 284,647, filed Aug. 29, 1972, that predilution techniques facilitated dewaxing of these heavy waxy oil stocks. It has now been found that even greater dewaxing efficiency can be obtained when predilution is combined with temporary periods of immiscibility in the dilution chilling operation involving these heavy types of feedstock. A more detailed description of the predilution techniques used in conjunction with the invention for dewaxing heavy waxy oil stock will be given below.

Any dewaxing solvent system containing two or more components of different polarity can be used as dewaxing solvents in the subject process. The solvent components may be premixed prior to introduction into the cooling zone or alternatively, individually introduced into the cooling zone and mixed therein. Mixtures of aliphatic ketones, containing three to eight carbon atoms, with each other, with aromatic compounds having from 6 to 12, preferably six or seven carbon atoms, and with alkenes of two to four carbon atoms may be used. Also, mixtures of aliphatic chloro compounds, having from one to four carbon atoms may be used. 11- lustrative of effective solvent compositions include toluene/benzene/acetone, benzene/acetone, methylethyl ketone (MEK)/toluene, acetone/methylisobutyl ketone (MIBK), MEK/MIBK, propylene/acetone, ethylenedichloride/dichloromethane and the like. Particularly preferred solvent compositions include MEK/toluene and MEK/MlBK.

As an example of how the process might be carried out in dilution chilling, with a toluene/MEK solvent system, the less miscible (more polar) solvent, MEK, is cooled and introduced into the first and successive stages of the tower and the oil is gradually cooled until a stage is reached in which there is bulk immiscibility between the solvent and oil. The more miscible (less polar) solvent toluene, is cooled and used in the next and successive stages until bulk miscibility is reattained. The two solvent streams are then used interchangeably to recreate this pattern of an immiscible stage followed by a miscible stage as is shown in more detail in the examples provided hereinbelow. An alternative embodiment would involve introducing a MEK- rich solvent stream into the first and successive stages, such as a 90/10 LV% MEK/toluene solvent system, and a toluene-rich solvent stream into the next and successive stages, such as a 10/90 LV% MEK/toluene solvent mixture. Thus, it is seen that the process of the subject invention may be operated in a number of different manners, accomplishing the same process objectives of producing temporarily substantially immiscible zones within the cooling zone.

It is noted that while it is preferred to accomplish most of the cooling of the oil by contact with cold dewaxing solvent, it is also contemplated that a substantial portion of the cooling may be provided by supplementary techniques such as scraped-surface chilling or autorefrigeration. Thus, for example, a propyleneacetone solvent could be used in conjunction with the dilution chilling process described above, with completion of chilling by autorefrigeration, wherein at least a portion of the propylene is vaporized thereby cooling the remaining oil.

The solvent mixture in a preferred operation of the subject process is prechilled to a temperature sufficient to permit the cooling of the oil to the desired temperature. It is apparent to those skilled in the art that the exact solvent temperature employed in the process will depend upon the amount of oil to be cooled and the amount ofsolvent to be added to the oil; i.e., the degree of dilution which is sought during the filtration step. The prechilled solvent is added incrementally along the length of the cooling zone, when dilution chilling dewaxing is practiced, so as to maintain an overall average cooling rate below about 10F per minute, more preferably below about 8F per minute and most preferably between 1 and 5F. per minute. In general, the amount of solvent added to the cooling zone will be sufficient to provide a liquid/solid weight ratio between the range of 10/1 and 150/1 at the dewaxing temperature and a solvent/oil volume ratio between about 1.0/1 and 7/1.

While the oil temperature may be reduced to the filtration temperature wholly within the dilution chilling zone, it is also contemplated to cool the oil to a temperature above the filtration temperature, to thereafter withdraw the wax oil solvent slurry formed therein, and accomplish the additional cooling to the filtration temperature in conventional dewaxing equipment such as scraped surface equipment. Operating in this manner provides not only more flexibility in the dewaxing operation but also offers substantial savings in refrigeration requirements and reduces the load of solvent recovery equipment.

As disclosed in U.S. Patent application Ser. No. 129,973, it is preferred that a high degree of agitation providing substantially instantaneous mixing of oil and solvent be provided in at least a portion of the stages in the dilution chilling operation, e.g., 1 second or less. These high levels of agitation are required only during the initial stages of crystal nucleation and growth. Once crystal development is well established, there is no longer any necessity for high agitation rates and lower agitation rates may be used.

The degree of agitation required in this function can be achieved when the modified Reynolds Number (Perry Chemical Engineers Handbook, 3rd Ed. p. 1224, McGraw-Hill, New York, 1959), N which is defined by the equation:

N Uni/p,

where L agitator diameter, ft. 1 liquid density, pound- /feet n agitator speed, r.p.s. ,u liquid viscosity, pound/feet second is between about 200 and about 100,000 and the dimensionless ratio of cooling tower diameter to agitator diameter is between about 1.5/1 and about lO/l. A flat-bladed turbine type agitator is preferred; however, other types of agitators such as propellers may be used.

As indicated above, the use of the process of this invention facilitates the dewaxing of heavy (high viscosity) petroleum oil stocks. These are oil stocks characterized by-having a viscosity greater than about SUS at 210F, with more than '10 percent of material boiling above about 800F. and more than 50 percent of material boiling below about 1,150F. In dewaxing such heavy oil stock, it is preferred that they first be subjected to a predilution process. A predilution process which is disclosed in co-pending U.S. patent application Ser. No. 284,647, filed Aug. 29, 1972, provides for the mixing of the heavy oil stock with at least about 0.3 volumes of a dewaxing solvent per volume of stock.

In one embodiment of the predilution process a waxy heavy oil stock is diluted with at least about 0.3 volumes of a predilution solvent per volume of oil stock, resulting in the depression of the cloud point of the oil stock. The cloud point of the oil is defined as the temperature at which a cloud or haze of wax crystals first appears when an oil is cooled under prescribed conditions (modified ASTM D2500-66 procedure). Predilution, as the term is used herein, refers to the mixing of solvent and oil prior to cooling of the oil to a temperature below its depressed cloud point.

The resultant solvent-oil mixture is introduced into a cooling zone divided into a plurality of stages, at a temperature above the depressed cloud point of the oil. Additional dewaxing solvent, which may be the same or different than the predilution solvent used to form the initial solvent-oil mixture, is introduced into at least a portion of the stages and high levels of agitation are maintained in at least a portion of the solventcontaining stages thereby providing efficient mixing of solvent and oil. The high levels of agitation referred to above are only necessary during the initial phases of wax crystal nucleation and growth. Once good crystal growth is effected, lower agitation levels may be used, e.g., in the later stages of the cooling zone.

The solvent-oil mixture is cooled as it passes through the cooling zone to a temperature below the depressed cloud point of the waxy oil stock, thereby precipitating at least a portion of the wax therefrom, and a residual oil stock of diminished wax content is recovered.

In another embodiment of the predilution process, the predilution of the oil is conducted in situ, i.e., within the cooling zone itself. To this end, the feedstock is introduced into the cooling zone at a temperature above its cloud point and in the substantial absence of solvent. At least about 0.3 volumes, more preferably, at least about 0.4 volumes of solvent per volume of oil is added to the initial stages of the cooling zone, coming into contact with the oil stock and forming an oil-solvent mixture. The mixture is gradually cooled, as it passes through the initial cooling stages, to a temperature no less than the depressed cloud point of the oil stock. Thereafter, additional solvent is introduced into at least a portion of the remaining stages of the cooling zone, and the oil is further cooled to a temperature below its depressed cloud point thereby precipitating at least a portion of the wax.

Although it is preferred that a substantial portion of the cooling of the oil be provided by the contacting of same with prechilled solvent, it is contemplated that other cooling means, such as autorefrigeration, wherein cooling is effected in part by vaporization of solvent, may also be employed.

The heavy feedstocks which are subjected to this predilution process contain the most difficulty vaporizable components of petroleum hydrocarbons including asphaltenes and pitch, which are undesirable not only in most finished products such as lubricating oil, but also in the intermediate refining operations. It is thus preferred, prior to the predilution process to remove as much of these components from the heavy stock as possible, such as by a deasphalting operation, e.g., propane deasphalting. Further, the heavy stock may contain aromatic and polar molecules which may be removed by using such process techniques as solvent extraction, comparatively severe hydrogen treatment and the like.

In general, the wax content of the feedstock as defined by the amount of material to be removed to produce an oil with a pour point in the range of +25 to 0F. will vary between about 10 to 30 wt. percent. The initial pour and cloud points of the oil will range, respectively, between about and 175F. and about and 180F.

The predilution solvent is selected from any of the dewaxing solvents known in the prior art such as the aliphatic ketones having from three to six carbon atoms, e.g., acetone, methylethyl ketone (MEK), methylisobutyl ketone (MIBK) and the like, the lower molecular weight hydrocarbons such as ethane, propane, butane and propylene, as well as mixtures of the foregoing ketones and mixtures of the ketones with hydrocarbon compounds such as propylene,'and aromatics such as benzene and toluene. In addition, halogenated low molecular weight hydrocarbons such as the C C chlorinated hydrocarbons, e.g., dichloromethane, dichloroethane and mixtures thereof, may be used. Specific examples of effective predilution solvents include toluene, MIBK, MEK/toluene, MEK/MIBK and the like. They may be the same as or different from the solvents used in the dewaxing operation of this invention.

The depressed cloud point of the oil is dependent, in part, upon the degree of predilution of the oil with solvent and will preferably range between about 50 and 175F., most preferably between about 50 and F. In general, the amount of predilution solvent added to the oil will be dependent, in part, on the nature of the feedstock, the cooling zone, the extent of cooling within the cooling zone, i.e., approach to the filtration temperature, and the desired final ratio of solvent to oil in the wax/oil/solvent slurry withdrawn from the cooling zone. Preferred amounts of predilution dewaxing solvent range between about 0.4 and 2.0 volumes per volume of residual oil stock, most preferred between about 0.5 to 15 volumes of solvent per volume of oil stock.

While the exact mechanism of the predilution process is not known, it is speculated that trace amounts of asphaltene and pitch components present in the heavy feedstocks, possibly as a result of contamination in the vacuum pipestill or inadequate deasphalting, interfere with the formation of wax crystals of the desired structure. Specifically, it is thought that at some temperature above the depressed cloud point of the feedstock, these asphaltene and pitch components precipitate from the oil as very small crystals which interfere with the uniform nucleation and growth of the wax crystals. It is contemplated that solvent predilution facilitates solution of these very small crystals, and delays their precipitation until after the cloud point temperature of the oil is reached, at which time, they cocrystallize with the wax components of the oil, thereby substantially reducing wax crystal growth interference.

It is also thought that predilution techniques in dilution chilling dewaxing reduce the overall viscosity of the oil stock in the critical early stages of crystal nucleation and growth thereby removing diffusion limitations to crystal growth and facilitating the development of larger particles.

This may be particularly important with the residual feedstocks and the like, since the wax crystallizing from such high boiling, high molecular weight feedstocks comprises highly branched paraffins and naphthenes, which have very low crystal growth rates. In contrast, the wax crystallizing from lower boiling distillate feedstocks generally contains predominantly normal paraffins, which have relatively high crystal growth rates and would therefore not be as sensitive to diffusion limitatrons.

Further information concerning the predilution of heavy petroleum oil feedstocks may be found in US. Patent application Ser. No. 284,647, filed on Aug. 29, 1972, the disclosure of which is hereby incorporated by reference. It is to be noted, in this connection, that the predilution process is applicable to the present invention only where the feedstock is a heavy petroleum oil feedstock as defined above, not where, for example, the feedstock is a lubricating oil fraction.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified flow scheme for a preferred embodiment of the dewaxing process of the subject invention.

FIG. 2 graphically demonstrates the use of miscibility data to determine operating conditions for temporarily immiscible dewaxing.

FIG. 3 is a simplified flow scheme showing the relationship between temperature variation and solvent compositionon miscibility within a given stage in a dilution chilling tower. The solvents used are pure methylethyl ketone and pure toluene.

FIG. 4 is similar to FIG. 3 except that the solvents are mixtures of methylethyl ketone and toluene in 90/10 and l/90 LV%.

FIGv 5 is similar to FIG. 3 except that the oil stock is prediluted with solvent prior to introduction into the dilution chilling tower.

FIG. 6 graphically demonstrates the beneficial results of the process with respect to DWO filter rate.

Referring to FIG. 1, in detail, a heavy waxy oil stock is introduced via line 11 into mixing zone 1. A dewaxing solvent, for example, methylethyl ketone (100 percent) is introduced into mixing zone 1 via line 13, and is admixed with the oil, to obtain a miscible composition. The resultant mixture is introduced into the first stage 29A of cooling tower 2, via line 15, at a temperature above the depressed cloud point of the oil. The cooling tower is divided into a plurality of stages 29, and at least a portion of the stages, preferably each of the stages, is provided with agitation means 27 for mixing solvent and oil. As indicated previously, the agitation means preferably comprises a rotating impeller and agitation levels providing substantially instantaneous mixing are desirable, at least in the early stages of the dewaxing process. In the displayed embodiment, the rotating impellers are driven by a variable speed motor 17, and the agitation is controlled by variation ofthe motor speed with due allowance for the flow rate through the cooling tower. While mechanical rotary type agitation is shown any mixing system can be employed in the process so long as it provides the high degree of mixing efficiency required for successful operation of the process. Solvents are introduced via lines 23 and 25 into manifolds 24 and 26 respectively. This results in the incremental addition of the solvent along the length of the cooling tower into at least a portion of the stages, preferably each of the stages in the chilling zone. For the purposes of the present discussion, the solvent introduced through line 23 comprises methylethyl ketone and the solvent introduced through line 25 comprises toluene. As indicated above, however, the solvents introduced through lines 23 and 25 may comprise more than one component respectively, such as MEK/toluene of two different ratios. Manifolds 24 and 26 comprise a series of parallel lines providing solvent. inlets l9 and 21, respectively, to the several stages of the cooling tower 2. The rate of flow through each inlet is regulated by a flow control means not shown. The rate of solvent flow is controlled so as to maintain a desired temperature gradient along the height of the coolingtower 2.

It is noted that while a vertical stage tower is shown, the shape of the cooling zone is not critical to the invention. The first portion of increment of solvent enters the first stage, i.e., 29A and is mixed with the predi luted oil, by means of rotating impeller blades 27A. The solvent mixture passes from stage to stage of the cooling tower and is gradually cooled therein. In the instant description, the introduction of cold dewaxing solvent provides substantially all of the cooling to the oil. The less miscible (more polar) solvent is introduced via line 23 into the cooling zone where it is mixed with the solvent-oil mixture already therein by means of rotating impeller blades 27A. The solvent is added to specific stages in the cooling tower and stages are eventually reached in which there is bulk immiscibility between solvent and oil. The more miscible (less polar) solvent is introduced via line 25 and inlet 21 into other stages of the cooling zone and bulk miscibility is re-attained in a subsequent stage. The two solvent streams are used interchangeably to recreate this pattern of an immiscible stage followed by a miscible stage. The final solvent composition is adjusted for miscibility or borderline miscibility at the filter temperature. The oil-solvent mixture can be cooled all the way down to the subsequent wax-oil separation temperature or alternatively, and preferably, to some temperature above the subsequent separation temperature. In the latter embodiment, the slurry removed from the cooling tower via line 31 can be further chilled (by means not shown) such as in scraped surface equipment, before entering separator 3. It is noted that the oil-solvent mixture may pass upwardly or downwardly through the cooling tower although downward flow has only been shown. Any number of stages may be used in the tower although it is preferred to use at least about six stages. As many as fifty stages may be employed in one embodiment of the invention. Preferably, about 16 or 17 stages are used although the invention could be conducted in six or less stages.

Any suitable separation means for separating wax from the slurry can be used such as filtration or centrifugation. The wax solvent mixture is removed from the separation zone 3 through line 35. The solvent is recovered in a suitable separating system 5, which is preferably distillation, through line 37 and the wax exits through line 39. The oil-solvent solution leaves the wax separation zone 3 through line 33 and passes to means for separating the oil from the solution 4. Any suitable means for the separation may be used, such as distillation or selective adsorption. The oil is removed from the separator and is recovered through line 41. The solvent is removed through line 43. The solvent may be recycled directly or scrubbed to remove impurities before reuse.

FIGS. 2 6 will be discussed in more detail below in connection with the specific embodiments of the invention.

PREFERRED EMBODIMENT In a preferred embodiment of this invention, the feedstock is a heavy (high viscosity) petroleum oil stock characterized by having a viscosity greater than about 100 SUS at 210F., with more than 10 percent of material boiling above 800F. and more than 50 percent of material boiling below about 1,150F. Prior to subjecting this feedstock to the process of this invention, it is subjected to the predilution process described more fully above. It is diluted with solvent in the range of from about 0.4 to about 1.0 volume of solvent per volume of feed.

The preferred solvents in the practice of this invention are methylethyl ketone and toluene. The predilution solvent and a solvent used as. the first dilution chilling solvent comprises methylethyl ketone and toluene in ratios of from about 100/0 to 85/15 LV%, i.e., the solvent consists entirely of or comprises a major portion of methylethyl ketone. For the second dilution chilling solvent, the ratios are reversed, i.e., the methylethyl ketone to toluene ratio is from /100 to 15/85 LV%. The temperature of the dilution chilling solvents are maintained at from about 30 to about 0F. Specifications for the cooling tower, agitator, baffles, etc. are those in conventional dilution chilling towers, such as those described in U.S. Pat. No. 3,681,230, the pertinent parts of which are herein incorporated by reference.

The first dilution chilling solvent is added to the prediluted feedstock in successive stages until a stage is reached in which there is bulk immiscibility, as determined by a liquid/liquid phase diagram forthe three component MEK/toluene/oil system. The use of the liquid/liquid phase data will be more easily understood from the examples which follow. The second dilution chilling solvent is then added to return the system to miscibility. This pattern of immiscibility, followed by miscibility if repeated until the desired final solvent composition of methylethyl ketone/toluene are from 40/60 to 60/40 LV% and the solvent/feed ratio of 2/1 to /1 is obtained. Further chilling is carried out in scraped surface equipment at separation temperatures of from about l0 to about +F.

The recovery system is a conventional recovery system, such as, for example, one of those described in the above reference U.S. Pat. No. 3,681,230.

The invention will be more apparent from the specific'examples set forth below.

EXAMPLE-l Referring to the accompanying FIG. 3 the dewaxing conditions for a feedstock having the following specifications Boiling range: less than 10 percent boiling below 900F. and more than 50 percent boiling below l-,l50F. at atmospheric pressure.

Gravity, API 27.7

Dry Wax Content 22 percent Viscosity at '2l0F 138 SU I Pour Point- F.

Cloud Point F can be determined in relation to a dilution chilling operation. For this feedstock, no predilution is required. Pure methylethyl ketone and pure toluene are used as the first and seconddilution chilling solvents, respectively, at temperatures of 20F. A final solvent composition of methylethyl ketone/toluene of 50/50 LV% is desired at a final solvent/feed ratio of 4/l. By heat balance, it can be calculated that for a feed entry temperature of F., the slurry exit temperature will be l8F. The solvent flow rates required for an equal temperature drop of 7F. for each of the 16 stages are shown in FIG. 3. Pure methylethyl ketone is used as the dilution chilling solvent in each stage until comparison of the stage temperature, solvent/oil ratio and solvent composition with the data shown in FIG. 2 indicates that there is bulk immiscibility in the stage. Then the pure toluene stream is used for dilution chilling, until comparison with the phase data in FIG. 2 indicates that miscibility is again reached. The streams are used interchangeably as shown in FIG. 3, until the desired final solvent composition ans solvent/feed ratio are obtained.

Filter rates by this technique will be approximately 20 to 50 percent higher than using a single premixed dilution chilling solvent, but yields of dewaxed oil are equivalent. Thus, this embodiment is of particular applicability where it is desired primarily to increase filtration rates.

EXAMPLE 2 This example shows the dewaxing of, a feedstock identical to that in Example I. The solvents, however, are mixed methylethyl ketone/toluene solvents. The first dilution chilling solvent comprises 90 percent methylethyl ketone and 10 percent toluene while the second dilution chilling solvent comprises 10 percent methylethyl ketone and 90 percent toluene. FIG. 4 is the simplified flow scheme showing the temperature variation and solvent composition.relationship.

I EXAMPLE 3 I This example shows the dewaxing of the feedstock of Example 1 using the same solvent system, but with the feedstock prediluted with 0.5 volumes of methylethyl ketone prior'to its introduction into the dilution chilling tower. As in Example 1, solvent composition and temperature are regulated so as to produce temporary periods of bulk immiscibility at given stages of the process, with interspersed periods of miscibility. The simplified flow diagram is shown in FIG. 5.

EXAMPLE 4 The utility of the process was demonstrated in a conventional incremental dilution dewaxing operation, which operation is described in more detail supra. Specifically, a Baytown 95 V.I. Bright Stock was dewaxed with MEK/toluene. One run was conducted using a constant solvent composition, i.e., MEK/toluene 53/47, while another run was conducted using the alternating immiscible-miscible sequence of the invention, this sequence attained by the use of split solvent addition. Specifically, the feed was prediluted with MEK prior to introduction into the cooling zone, followed by incremental addition of tolueneduring the course of the 13 14. cooling operation. Operating conditions are displayed ing polarity, the more polar solvent being less miscible in Table I below. The variation of the miscible and imwith the oil than the less polarsolvent. said solvents miscible phase regimes is shown in FIG. 2 (dotted line). being introduced into at least a portion of the stages of 1n each case, as shown in FIG. 6, at a given agitator the cooling zone in incremental amounts, adjusting the speed, split solvent addition gave better performance 5 composition and amounts of said solvents and graduthan addition of increments ofconstant solvent compoally reducing the temperature in said cooling zone, sition. v thereby pr oducing inat least a portion of the stages in TABLE 1* Increment Total Dilution Temp. (F.) (Solvent/Oil) (Solvent/Oil) MEK TOL Predilution 150 2.65/1 2.65/1 100 11% .85/1 5.00/1 53 47 Filter temperature 9-10 I Filtratio n lS cm. Buchner Equal wash, filter times What is claimed is: said cooling zone, a plurality of temporary periods of 1. A process for separating wax from a waxy petrobulk immiscibility between said solvent system and oil, leum oil stock comprising the steps of introducing said interspersed with temporary periods of bulk miscibility oil stock, at a temperature above its cloud point, into between said solvent system and oil, cooling and agitata cooling zone, said oil containing at least a portion of ing the oil and solvents as they pass through the cooling the wax dissolved therein, contacting said oil as it zone, thereby precipitating at least a portion ofthe wax passes through the cooling zone with a dewaxing 501- from the oil and forming a solvent/oil mixture containvent system comprising two or more solvents of differing precipitated wax, and recovering oil of diminished ing polarity, the more polar solvent being less miscible wax content. with the oil than the less polar solvent, said solvents 8, A cess a cording t claim 7 in which the oil being introduced 11110 the cooling ZOIIE 11'1 incremental tock is ooled by contacting it with cold dewaxing so]- amounts, adjusting the composition and amounts of v t said solvents and gradually reducing the temperature in 9, A ro es a ording to laim "7 in whi h the oil the cooling zone, thereby Pro uc ng therein a p urality stock is mixed with a dewaxing solvent prior to introof temporary periods of bulk immiscibility between d i i i h li Zone, said solvent system and oil interspersed with temporary 10, A process according to l i 7 h i a hi h d Periods of bulk miscibility between Said Solvent System gree of agitation is provided in at least a portion of said and oil, cooling and agi at g e Oil and Solvents a5 stages so as to effect substantially instantaneous mixing they pass through the cooling zone, thereby precipitatf id solvents d id il. ing at least a portion of the wax from said oil and form- 11 A process according to l i 10 i hi h h 1- ing a Solvent/Oil mixture Containing Precipitated Wax, 4O vent system comprises two components, the first comand recovering Oil of diminished Wax Come! ponent being an aliphatic ketone of from three to eight 2- A prOCSS according to l in the S01- arbon atoms and the econd component being Se. Vent System Comprises an aliphatic ketone containlected from the group consisting of an aliphatic ketone ing from three to eight carbon atoms and (b) a solvent f f three to eight Cal-hon atoms d h i a l Selected from the group consisting of an aliphatic ity different from the ketone of the first component, an t0m3 Containing from three to eight Carbon atoms and aromatic compound of from six to 12 carbon atoms, an having [655 P y than Component an aromatic alkene of from two to four carbon atoms or mixtures compound having from 6 to 12 carbon atoms, an alh f kehe Containing from two to four Carbon atoms and 12. A process according to claim 11 in which the first mixtures thereof. component is a dialkyl ketone of from three to six car- 3. A process according to a m 2 in Which the bon atoms and the second component is an aromatic vent system comprises (a) a dialkyl ketone having from v Compound f i or Seven carbon atoms three to eight Carbon atoms and an aromatic 13. A process according to claim 12 in which the first pound having seven carbon atOm component is methylethyl ketone and the second com- 4. A process according to claim 3 in which the solponent i tolucnc Vent System Comprises methylethyl ketone and Q1 14. A process according to claim 13 in which the oil The Process according to claim 2 wherein the stock is mixed with methylethyl ketone prior to intro- Veht System Comprises methylethyl ketone and ducing it into the cooling zone and in which the methylthylisobutyl ketone. 0 ethyl ketone and toluene are individually introduced in The Process according to Claim 2 wherein the varying proportions into at least a portion of the stages vent system comprises propylene and acetone. i id li Zone,

A Process for the dewaxihg of a y Petroleum 15. The process according to claim 15 in which the oil stock comprising the steps of introducing Said oil stock is mixed with a solvent comprising from about SIOCk into a Cooling Zone divided into a plurality of -100 percent methylethyl ketone and from about 15 Stages contacting said 011 Stock as it passes from Stage to 0 percent of toluene prior to introducing said oil t0 Stage thr g Said cooling Zone with a dewaxihg stock into said cooling zone, and in which methylethyl vent system comprising two or more solvents of differk t d t l are i d i varying proportions prior to their introduction into at least a portion of the stages of said cooling zone.

16. The process according to claim 7 in which the waxy petroleum oil stock is a heavy residual petroleum oil stock having a viscosity greater than about 100 SUS at 210F, with more than 10 percent thereof boiling above about 800F and more than 50 percent thereof boiling below about l,l50F.

17. A process according to claim 16 in which the heavy petroleum stock is diluted with at least about 0.3 volumes of a dewaxing solvent per volume of oil feedstock, prior to its introduction into the cooling zone, thereby depressing the cloud point of said oil stock.

18. A process according to claim 17 in which the oil stock is diluted with from about 0.4 volumes of solvent to about two volumes of solvent per volume of oil stock.

19. The process according to claim 17 in which the solvent is a dialkyl ketone of from three to six carbon atoms.

20. The process of claim 19 in which the solvent is methylethyl ketone or methylisobutyl ketone.

21. A process according to claim 16 wherein said oil stock is introduced into said cooling zone at a temperature above its cloud point and, in the initial stages of said cooling zone, contacted with at least about 0.3 volumes of solvent per volume of oil stock, thereby depressing the cloud point of said oil stock and forming a solvent/oil mixture, cooling said mixture as it passes through the initial stages of said cooling zone, to a temperature no lower than the depressed cloud point of the oil, introducing dewaxing solvent into at least a portion of the remaining stages of said cooling zone and cooling the solvent oil mixture as it passes through said remaining stages to a temperature below its depressed cloud point thereby precipitating at least a portion of the wax from said oil.

22. The process of claim 1 wherein a first solvent is added to the oil in the cooling zone and then a second solvent of different polarity is later added to said oil containing said first solvent in the cooling zone.

23. A process according to claim 1 wherein a high degree of agitation is provided in at least a portion of said cooling zone so as to effect substantially the instantaneous mixing of said solvents and said waxy petroleum oil stock.

24. The process of claim- 7 wherein a first solvent is added to'the oil in a cooling zone and then a second solvent of different polarity is later added to said oil containing said first solvent in the cooling zone. 

1. A PROCESS FOR SEPARATING WAX FROM A WAXY PETROLEUM OIL STOCK COMPRISING THE STEPS OF INTRODUCING SAID OIL STOCK, AT A TEMPERATURE ABPVE ITS CLOUD POINT, INTO A COOLING ZONE, SAID OIL CONTAINING AT LEAST A PORTION OF THE WAX DISSOLVED THEREIN, CONTACTING SAID OIL AS IT PASSES THROUGH THE COOLING ZONE WITH A DEWAXING SOLVENT SYSTEM COMPRISING TWO OR MORE SOLVENTS OF DIFFERING POLARITY, THE MORE POLAR SOLVENT BEING LESS MISCIBLE WITH THE OIL THAN THE LESS POLAR SOLVENT, SAID SOLVENTS BEING INTRODUCED INTO THE COLLING ZONE IN INCREMENTAL AMOUNTS, ADJUSTING THE COMPOSITION AND AMOUNTS OF SAID SOLVENTS AND GRADUALLY REDUCING THE TEMPERATURE IN THE COOLING ZONE, THEREBY PRODUCING THEREIN A PLURALITY OF TEMPORARY PERIODS OF BULK IMMISCIBILITY BETWEEN SAID SOLVENT SYSTEM AND OIL INTERSPERSED WITH TEMPORARY PERIODS OF BULK MISCIBILITY BETWEEN SAID SOLVENT SYSTEM AND OIL, COOLING AND AGITATING THE OIL AND SOLVENTS AS THEY PASS THROUGH THE COOLING ZONE, THEREBY PRECIPITATING AT LEAST A PORTION OF THE WAX FROM SAID OIL AND FORMING A SOLVENT/OIL MIXTURE CONTAINING PRECIPITATED WAX, AND RECOVERING OIL OF DIMINISHED WAX CONTENT.
 2. A process according to claim 1 in which the solvent system comprises (a) an aliphatic ketone containing from three to eight carbon atoms and (b) a solvent selected from the group consisting of an aliphatic ketone containing from three to eight carbon atoms and having less polarity than component (a), an aromatic compound having from 6 to 12 carbon atoms, an alkene containing from two to four carbon atoms and mixtures thereof.
 3. A process according to claim 2 in which the solvent system comprises (a) a dialkyl ketone having from three to eight carbon atoms and (b) an aromatic compound having seven carbon atoms.
 4. A process according to claim 3 in which the solvent system comprises methylethyl ketone and toluene.
 5. The process according to claim 2 wherein the solvent system comprises methylethyl ketone and methylisobutyl ketone.
 6. The process according to claim 2 wherein the solvent system comprises propylene and acetone.
 7. A process for the dewaxing of a waxy petroleum oil stock comprising the steps of introducing said oil stock into a cooling zone divided into a plurality of stages, contacting said oil stock as it passes from stage to stage through said cooling zone with a dewaxing solvent system comprising two or more solvents of differing polarity, the more polar solvent being less miscible with the oil than the less polar solvent, said solvents being introduced into at least a portion of the stages of the cooling zone in incremental amounts, adjusting the composition and amounts of said solvents and gradually reducing the temperature in said cooling zone, thereby producing in at least a portiOn of the stages in said cooling zone, a plurality of temporary periods of bulk immiscibility between said solvent system and oil, interspersed with temporary periods of bulk miscibility between said solvent system and oil, cooling and agitating the oil and solvents as they pass through the cooling zone, thereby precipitating at least a portion of the wax from the oil and forming a solvent/oil mixture containing precipitated wax, and recovering oil of diminished wax content.
 8. A process according to claim 7 in which the oil stock is cooled by contacting it with cold dewaxing solvents.
 9. A process according to claim 7 in which the oil stock is mixed with a dewaxing solvent prior to introducing it into the cooling zone.
 10. A process according to claim 7 wherein a high degree of agitation is provided in at least a portion of said stages so as to effect substantially instantaneous mixing of said solvents and said oil.
 11. A process according to claim 10 in which the solvent system comprises two components, the first component being an aliphatic ketone of from three to eight carbon atoms, and the second component being selected from the group consisting of an aliphatic ketone of from three to eight carbon atoms and having a polarity different from the ketone of the first component, an aromatic compound of from six to 12 carbon atoms, an alkene of from two to four carbon atoms or mixtures thereof.
 12. A process according to claim 11 in which the first component is a dialkyl ketone of from three to six carbon atoms and the second component is an aromatic compound of six or seven carbon atoms.
 13. A process according to claim 12 in which the first component is methylethyl ketone and the second component is toluene.
 14. A process according to claim 13 in which the oil stock is mixed with methylethyl ketone prior to introducing it into the cooling zone and in which the methylethyl ketone and toluene are individually introduced in varying proportions into at least a portion of the stages in said cooling zone.
 15. The process according to claim 15 in which the oil stock is mixed with a solvent comprising from about 85-100 percent methylethyl ketone and from about 15 to 0 percent of toluene prior to introducing said oil stock into said cooling zone, and in which methylethyl ketone and toluene are premixed in varying proportions prior to their introduction into at least a portion of the stages of said cooling zone.
 16. The process according to claim 7 in which the waxy petroleum oil stock is a heavy residual petroleum oil stock having a viscosity greater than about 100 SUS at 210*F, with more than 10 percent thereof boiling above about 800*F and more than 50 percent thereof boiling below about 1,150*F.
 17. A process according to claim 16 in which the heavy petroleum stock is diluted with at least about 0.3 volumes of a dewaxing solvent per volume of oil feedstock, prior to its introduction into the cooling zone, thereby depressing the cloud point of said oil stock.
 18. A process according to claim 17 in which the oil stock is diluted with from about 0.4 volumes of solvent to about two volumes of solvent per volume of oil stock.
 19. The process according to claim 17 in which the solvent is a dialkyl ketone of from three to six carbon atoms.
 20. The process of claim 19 in which the solvent is methylethyl ketone or methylisobutyl ketone.
 21. A process according to claim 16 wherein said oil stock is introduced into said cooling zone at a temperature above its cloud point and, in the initial stages of said cooling zone, contacted with at least about 0.3 volumes of solvent per volume of oil stock, thereby depressing the cloud point of said oil stock and forming a solvent/oil mixture, cooling said mixture as it passes through the initial stages of said cooling zone, to a temperature no lower than the depressed cloud point of the oil, introducing dewaxing soLvent into at least a portion of the remaining stages of said cooling zone and cooling the solvent oil mixture as it passes through said remaining stages to a temperature below its depressed cloud point thereby precipitating at least a portion of the wax from said oil.
 22. The process of claim 1 wherein a first solvent is added to the oil in the cooling zone and then a second solvent of different polarity is later added to said oil containing said first solvent in the cooling zone.
 23. A process according to claim 1 wherein a high degree of agitation is provided in at least a portion of said cooling zone so as to effect substantially the instantaneous mixing of said solvents and said waxy petroleum oil stock.
 24. The process of claim 7 wherein a first solvent is added to the oil in a cooling zone and then a second solvent of different polarity is later added to said oil containing said first solvent in the cooling zone. 